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During discussion about emergency landing on water in this YouTube video (4:00) the flight instructor said:
"...the problem with these airplanes with a fixed gear, you're usually going to be upside down".
Is it really like this? If so, is there any water landing technique which could minimize the risk?
Depends on the airplane. I recall reading about a guy flying across from the Aleutians to Japan, in a Piper Super Cub of all things, and he had to ditch it for whatever reason, which he was able to do next to a freighter.
To his surprise, it didn't flip over but kind of submarined, stopped, then popped to the surface nose down but upright. It's possible being a taildragger helped. I think tricycle gear airplanes would be more prone to flipping right over because the pitching moment from water contact is a little stronger with 3 gears hanging down vs two.
I expect though, that if you surveyed ditchings you would find some do and some don't. Pitching moment from water contact, elevator effectiveness in resisting the pitching, amount of up elevator at water contact, speed at water contact, maybe the surface contours of the fuselage could promote it or inhibit it, all kinds of variables.
As xxavier says, it would be prudent to assume you are going to flip over, and if you don't, bonus.
It is situational. Locally there have been three Cessna ditchings that I have first hand knowledge of. One of them resulted in an inversion. The other two did not.
The general wisdom seems to be that high wing aircraft are more likely to upset than low wing aircraft. I do not know what NSTB statistics are.
I am sure there are factors such as: vertical CG, AOA and or the attitide hitting the water, glassy water vs rough conditions, wind, gear dressings (tires, pants) etc. At one time, probably 18 or so years ago, I saw a aeronautical engineering student's work on a simulation to model the likelihood of upsets using various commuter and short haul aircraft models, and what might happen in a ditching. I have not seen anything more definitive since.
In the interim, if you plan on over water flights, the FAA safety seminar where one practices egress inverted and underwater, is a nice seminar. I have taken it twice, and I learn as much the second time around as I did the first. Fortunately, I have only had to demonstrate my skills in a high school swimming pool.
Not all water is as friendly as the Hudson during daylight hours.
The answer is in the torque created by protruding landing gear when they strike the water first with any forward motion relative to the landing surface. Water is much denser than air, so the drag created at the end of the lever arm of the landing gear easily flips the plane over.
Thus is why, if possible, water ditching is done gear up with the intent of "belly flopping" the aircraft onto the water. Low wings and fuselage can absorb the impact over a greater area and will generally have a lower center of gravity, but GREAT CARE must be taken not to catch a wing tip first as this (also by torque) will cartwheel the aircraft. High wings also stand a greater chance of remaining upright with gear up, if possible.
The best way is to reduce the ground (water) speed as much as possible by landing into the wind and holding the nose up as long as possible, applying full up elevator on contact. Full flaps should be used for the lowest landing speed possible.
As @ mongo said, training to escape an inverted aircraft in water is great life insurance, and would be worth the time in any type of aircraft.
In theory, for small airplanes with a very low stall speed, and with the help of a kind headwind, you may slow the plane down so much that the impact with the water may be almost vertical. But that's the theory... In practice, you should be ready to exit the cockpit inverted and underwater...
